1. Field of the Invention
This invention relates to the wire bonding technology in general. More specifically, the present invention relates to a method and apparatus for electrically connecting between two different portions of a workpiece (e.g. a leadframe or printed circuit board to which electronic components are mounted) by a solder wire.
2. Description of the Prior Art
In electronic circuits or components, it is usually necessary to electrically connect between two different portions of a workpiece. For instance, a lead of a semiconductor chip device need be electrically connected to a relevant bonding pad of the chip body through a metal wire.
In wire bonding, generally, a gold or aluminum wire is melted at both ends to form balls which are subsequently utilized for strong attachment to two different portions of a workpiece. Such a wire bonding method, which is called "ball bonding", is possible because either gold or aluminum has a high melting point and is reluctantly oxidized.
If the melting point of a metal wire is low, it is difficult to control ball formation (with respect to the ball diameter for example) at the time of thermally cutting a material metal wire by a gas torch (burning with hydrogen gas for example). Further, if the metal wire is easily oxidized at the time of ball formation, the metal wire fails to provide a sufficiently strong and reliable connection to the workpiece.
On the other hand, it has been recently proposed to use a solder wire for wire bonding in place of a gold or aluminum wire, because solder is known to be less costly than either gold or aluminum. Further, a solder wire is also advantageous in that it can be made to have an additional function as a temperature fuse (or as a combined temperature/overcurrent fuse) since the melting point of solder is relatively low.
However, the low melting point of solder makes it difficult to control ball formation at both ends of a solder wire. Further, solder is known to be oxidized very easily at the time of melting. Thus, it has been believed impractical to applying the ball bonding method to the solder wire.
In a wire bonding method using a solder wire, therefore, two different methods are conventionally used for bonding the solder wire. A first method is the soldering method wherein a separate bonding solder layer is formed between each end of the solder wire and a relevant portion of a workpiece. A second method is the so-called "wedge bonding" method wherein each end of the solder wire is compressed by a bonding tool for flattening at the time of bonding to the workpiece.
However, the soldering method requires the use of solder in addition to the solder wire itself to result in a material waste. More importantly, the wire bonding process cannot be performed quickly and efficiently by the soldering method. Further, the soldering method cannot be applied when the interval between the bonding points is narrow.
On the other hand, the wedge bonding method has no problem of material waste and is applicable even when the interval between the bonding points is narrow. However, compared with the ball bonding method, there is a greater limitation in increasing the bonding area by flattening the wire end, so that it is difficult to obtain a sufficient bonding strength. Further, the flattened end portion of the wire is rather thin and easy to break, resulting in a quality deterioration of the products.